Answer to Question 1
Water destroys particle cohesion by filling pores in sediment (saturation), which triggers mass wasting. However, when water only partially fills the pore spaces, it actually increases cohesion. Water also adds mass to the material, and this mass alone may be sufficient to cause gravity to act on a slope.
Answer to Question 2
Weathering is the physical breakdown and chemical alteration of rock at or near Earth's surface. Mass wasting is the transfer of rock and/or soil downslope due to gravity. Erosion is the physical removal of material by a mobile agent, such as flowing water, eaves, wind, or glacial ice.